There is an inherent balance problem in push-pull switching transistor controlled square wave converters. Should the circuit become unbalanced, one of the transistors conducts longer than the other, and the transformer core becomes saturated. This causes large peak currents to appear in the transistors. This condition is normally avoided by monitoring the current through the transistors, and if the current in either transistor exceeds a particular maximum, to terminate the drive pulse to that transistor so that the transformer core does not saturate.
Automatic self-balancing is achieved in the system of the invention so as to avoid such core saturation, and this is achieved in a simple and expeditious manner. As mentioned above, in accordance with the invention, a load dependent ripple voltage which appears on the output of the pre-regulator chopper is used to terminate the drive pulse to either transistor should the current through that transistor exceed a particular maximum.
The pre-regulator chopper has a finite impedance which causes a ripple voltage to be superimposed on its output which is dependent on the current flow through either of the transistors. In the practice of the invention, this ripple voltage is introduced to the square wave oscillator driving the transistors, and it serves to terminate the drive pulse from the oscillator to either one of the transistors when the current through that transistor exceeds a predetermined maximum.